1. Field of the Invention
This invention relates to automatic brightness limiter (ABL) circuits and particularly to a combination of ABL circuits, one of which is responsive to brightness changes at a lower level than another but requiring the brightness changes to which it responds to last longer than the higher level changes to which the other ABL circuit responds.
2. The Prior Art
In a television receiver, it is desirable to limit the brightness of images produced on the screen of a cathode ray tube. The brightness is related to the anode current of the cathode ray tube, and this current must be produced by a high voltage power supply. The high voltage power supply may be damaged if it is required to supply too high a current for too long a time. Also, the high anode current may damage the fluorescent screen of the cathode ray tube. Whether or not there is any damage to the power supply or the cathode ray tube, excessively bright images may cause the viewers to feel eyestrain. In the form of sudden flashes that cause no other problem, they may simply be objectionable to the viewer.
The foregoing disadvantages of unnecessarily high brightness levels have been recognized in the past, and ABL circuits have been provided to control the brightness, especially in color television receivers. Since the brightness is proportional to the anode current of the cathode ray tube, such ABL circuits have usually been coupled to a circuit through which anode current of the cathode ray tube flows and have responded by controlling or limiting the luminance signal when the anode current rose above a threshold level.
It is desirable theoretically that an ABL circuit respond without delay, but actually such an ABL circuit is not possible. One reason it is not possible is that the anode current is not continuous but drops to zero during each horizontal and vertical blanking interval. Thus, even in the simplest form, the anode current may be considered to approximate a pulse wave. If the ABL circuit would be responsive to such rapid changes of anode current, or brightness, it could cause the quality of the reproduced television images to be deteriorated. Such deterioration could appear as blurred areas of darkness and light.
Therefore, it has been customary to design prior art ABL circuits to respond only to brightness levels so that only changes in brightness levels that are slower than the vertical scanning period are detected and accounted for. The time constant of an ABL circuit may have a value of 50 to 60 milli-seconds, which is about three or four times the duration of a vertical scanning period, so that the ABL circuit is not influenced by a change in brightness that lasts for only a single line scanning interval or less, or even by a change in brightness that lasts for a single vertical scanning interval. Such an ABL circuit is referred to as an averaging-type ABL circuit.
Averaging-type ABL circuits have several disadvantages. They cannot respond to rapid or momentary brightness changes, such as occur when the tuner, or channel selector, of a receiver is actuated to shift from one operating channel to another. When the tuner passes through unused frequency bands, the screen of the cathode ray tube may receive momentary flashes of high brightness. The period for passing through such unused frequency bands is usually short and is in the order of 20 milli-seconds, so that ABL circuits with time constants of 50 milli-seconds or more do not have time to respond. Other flashes of brightness too short to be controlled by averaging-type ABL circuits occur, for example, when the main power switch is actuated and when there are troubles in the driving circuits for the cathode ray tube.
It is an object of the present invention to provide an automatic brightness limiter circuit capable of responding quickly to limit sudden large brightness increases in an image display device and more slowly to limit brightness increases of lesser intensity.
It is another object of this invention to provide a novel automatic brightness control circuit in which first and second ABL circuits are utilized.
It is a further object of this invention to provide an improved ABL circuit which is responsive to a rapid or momentary change in brightness of images reproduced on the screen of a cathode ray tube.
Another object of the invention is to provide a combination of two automatic brightness limiter circuits coupled to a brightness current circuit, the first limiter circuit responding to brightness current in excess of a first threshold level and comprising a first time constant circuit to limit to a first value the maximum frequency to which the first limiter circuit can respond, and the second limiter circuit responding to brightness current only in excess of a higher threshold level than the first threshold level and comprising a second time constant circuit having a lower time constant value than the first time constant circuit to be responsive to frequencies higher than the maximum response frequency of the first time constant circuit.